Reduced Flow Impacts Salmonid Smolt Emigration in a River 2 with Low-Head Weirs
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Durham Research Online Deposited in DRO: 06 April 2016 Version of attached le: Accepted Version Peer-review status of attached le: Peer-reviewed Citation for published item: Gauld, N.R. and Campbell, R.N.B. and Lucas, M.C. (2013) 'Reduced ow impacts salmonid smolt emigration in a river with low-head weirs.', Science of the total environment., 458-460 . pp. 435-443. Further information on publisher's website: http://dx.doi.org/10.1016/j.scitotenv.2013.04.063 Publisher's copyright statement: NOTICE: this is the author's version of a work that was accepted for publication in Science of The Total Environment. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reected in this document. Changes may have been made to this work since it was submitted for publication. 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Durham University Library, Stockton Road, Durham DH1 3LY, United Kingdom Tel : +44 (0)191 334 3042 | Fax : +44 (0)191 334 2971 https://dro.dur.ac.uk 1 Reduced flow impacts salmonid smolt emigration in a river 2 with low-head weirs 3 4 Final version accepted for publication in Science of the Total Environment 5 6 N. R. Gaulda,b, R. N. B. Campbellb, M. C. Lucasa 7 8 a School of Biological and Biomedical Sciences, Durham University, South Road, Durham, UK, DH1 9 3LE 10 b The Tweed Foundation, Drygrange Steading, Melrose, UK, TD6 9DJ 11 Corresponding author: N. R. Gauld, School of Biological and Biomedical Sciences, Durham University, 12 South Road, Durham, UK, DH1 3LE 13 Email address: [email protected] 14 15 Abstract 16 The impacts of large dams on the hydrology and ecology of river systems are well 17 understood, yet the impacts of low-head structures are poorly known. While impacts of small weirs 18 on upstream-migrating fish have long been mitigated by fish ladders, it is assumed that downstream 19 migration of surface-oriented fishes is unaffected under natural flow regimes. To test this, the 20 effects of low-head weirs and the influence of river flow on the migration of brown trout (Salmo 21 trutta) smolts in the River Tweed, UK, was examined. Movements of acoustic tagged smolts were 22 quantified in 2010 and 2011 using automatic listening stations and manual tracking throughout the 23 migration route. In both years smolts exhibited major losses, most likely due to predators, with 24 escapement rates of 19% in 2010 and 45% in 2011. Loss rates were greater in 2010 when flows were 25 frequently below Q95 (20% of study period), compared to 2011 when more typical flows 26 predominated (0% of study period below Q95). Smolts experienced significantly longer delay at 27 weirs during 2010 than 2011, associated with the different hydrographs during emigration as well as 28 weir design. Flow comparisons within the study periods and historical records shows that low flows 29 experienced in 2010 were not unusual. The swimming behaviour of smolts in relation to flow 30 conditions differed between years, with smolts in 2010 increasing their rate of movement in relation 31 to increasing flow at a faster rate than smolts in 2011. This is the first study to demonstrate river 32 flow impacts on the migration success of wild salmonid smolts at small weirs. Because small weirs 33 are common in rivers and because spring-summer low flow periods may become more frequent with 34 climate change (based on UKCIP09 models) and altered river hydrology, further research and 35 improved management is needed to reduce the impacts of low river flows in combination with low- 36 head weirs on salmonid smolt migration. 37 Keywords: Salmo trutta, smolt migration, habitat fragmentation, river obstructions, low flow 38 1. Introduction 39 In many developed countries there is a long history of river modification and, as a result, in-river 40 structures such as dams and weirs are present in half of the world’s rivers (Dynesius and Nilsson, 41 1994; Nilsson et al., 2005). Such modification has been integral to human population growth through 42 processes such as flood defence; power generation and farming in floodplains (Nilsson et al., 2005; 43 Poff and Hart, 2002). However, in-river barriers such as dams and weirs have a major role in the 44 fragmentation of fluvial ecosystems (Dynesius and Nilsson, 1994; Fullerton et al., 2010; Jungwirth, 45 1998; Kemp and O'Hanley, 2010). In-river barriers can have major impacts on fish populations by 46 preventing or restricting movement to habitats required for essential stages of fish life history 47 (Branco et al., 2012; Lucas and Baras, 2001; Lucas and Batley, 1996; Lucas et al., 2009; Wollebaek et 48 al., 2011). In-river barriers not only impact fish populations by restricting essential movement, there 49 is also major impacts on fish habitat due to alteration of the downstream flux of water and 50 sediment, nutrient movement, and water temperatures within rivers (Poff and Hart, 2002). The 51 effects of migration obstacles depend on factors such as fish species; river hydrology and barrier 52 type, with effects varying from short delays to complete blockage (Kemp and O'Hanley, 2010; 53 Northcote, 1998). In Europe, legislation such as the Water Framework Directive (WFD; 2000/60/EC) 54 requires free passage for migratory fish travelling between areas of river essential for their life 55 history, such as juvenile emigration from natal areas and adult spawning migrations. Failure to 56 comply can result in the river being assigned less than “Good ecological status” and may result in 57 sanctions. 58 The seaward migration of juvenile anadromous salmonids (smolts) is a crucial event in their 59 life history. Smoltification is a period of great morphological, behavioural and physiological change 60 when juvenile salmonids develop various adaptations that enable them to survive at sea (Debowski 61 et al., 1999a; Debowski et al., 1999b; Denton and Saunders, 1972; Lysfjord and Staurnes, 1998; 62 McCormick et al., 1998). The smolt migratory period is precisely timed with photoperiod, river 63 discharge and temperature playing determinate roles in its commencement (Björnsson et al., 1995; 64 Björnsson et al., 2010; McCormick, 1994; McCormick et al., 2000; McCormick et al., 2007; 65 McCormick et al., 2002). Throughout migration smolts are subject to elevated predation risk from 66 mammalian; avian and fish predators (Aarestrup et al., 1999; Aarestrup and Koed, 2003; Carss et al., 67 1990; Dieperink et al., 2002; Dieperink et al., 2001; Harris et al., 2008; Heggenes and Borgstrom, 68 1988; Koed et al., 2002; Steinmetz et al., 2003; Svenning et al., 2005a; Svenning et al., 2005b; Wiese 69 et al., 2008). Delays at river obstructions during such a timing-specific and vulnerable life history 70 stage can potentially have large impacts on the survival of smolts and the health of salmonid stocks 71 as a whole. 72 The impacts of large dams on the hydrology and ecology of temperate river systems, 73 including downstream fish passage, especially of economically important salmonids, are relatively 74 well known. In general downstream salmonid passage efficiency over dams is high (74.6%) based on 75 recent quantitative assessment (Noonan et al., 2012). However, high smolt mortalities due to both 76 physical damage and predation have been observed at major impoundments and hydro-power 77 facilities (Aarestrup et al., 1999; Hockersmith et al., 2003; Keefer et al., 2012; Muir et al., 2001a; 78 Muir et al., 2001b; Raymond, 1979; Raymond, 1988; Smith et al., 2006; Smith et al., 2002; Williams 79 et al., 2001). Low flows due to regulation in river reaches also cause delays in smolt emigration and 80 result in increased duration of exposure to mortality risks (Aarestrup and Koed, 2003; Keefer et al., 81 2012). However, the impacts of low-head structures, such as simple overflow weirs are poorly 82 known for downstream migrants (Lucas and Baras, 2001) with the exception of bottom-orientated 83 freshwater eels (Acou et al., 2008). While impacts of small weirs on upstream-migrating fish (Lucas 84 and Frear, 1997; Ovidio and Philippart, 2002) have been partially mitigated by fish ladders designed 85 specifically to assist upstream passage (Clay, 1995), average passage efficiencies are relatively low 86 (41.7%) (Noonan et al., 2012) and presence of passage fascilities is not always guaranteed to 87 mitigate passage concerns (Roscoe and Hitch, 2010). However, it is generally assumed that 88 downstream migration of wild surface-oriented fishes such as salmonid smolts is relatively 89 unaffected and that they will pass simple overflowing weirs unhindered under reasonably natural 90 flow regimes (Lucas and Baras, 2001). Some studies on passage of hatchery-reared smolts past small 91 weirs, in particular that of Aarestrup and Koed (2003), strongly contradict this. To test this 92 assumption for wild fish, the effects of low-head weirs and the influence of natural variations in river 93 flow on the migration behaviour and survival of anadromous brown trout (Salmo trutta) smolts were 94 examined in the River Tweed, UK, a catchment with very strong wild migratory salmonid stocks.